There is a veterinary nutritionist -Dr. Marge Chandler -here:
http://www.vetsnowreferrals.com/veterinary-referral-swindon/nutrition-adivce-swindon/
But this looks like the UK. However she may do consultations online...?
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The American College of Veterinary Nutrition here:
http://www.acvn.org/about-us/faq/
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Ohio State University Veterinary Medical Center....nutrition support service, here:
http://vet.osu.edu/nssvet
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PetDIETS.com
https://www.petdiets.com/
for some general info and an option to contact the nutritionist.
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If you do decide to do a home cooed regime for Bailey you must let your vet know. In fact you may even need a referral to consult a nutritionist face to face.
I feel your concern. I would be concerned too. The vet's idea doesn't seem to be working, although I can kind of go along with the idea that a form of diet is a wise thing generally.
Anyway, this is what Hills diabetic food contains:
ground whole grain corn, powdered cellulose, peanut hulls, chicken by-product meal, chicken liver flavour, soybean meal, soybean mill run, dried egg product, soybean oil, corn gluten meal.... (plus vitamins etc)
This sounds weird. "Peanut hulls"??? Since when were peanut hulls good basic food for dogs?? And the rest of it sounds like junk from dubious origins.
The other brands of prescription foods fare no better, in my opinion.
I know what I would do if I were you. I would put everything I have into a consultation with a veterinary nutritionist -if you possibly can, And work out a home-cooked diet based on advice from him/her. Some intensive research online may help...
It will be hard work I am sure and may be confusing sometimes, but I think that is the way I would go.
Sorry to project my own opinions....